Certain bacteria can be harmful or even deadly to humans as well as animals. In September 2001, anthrax spores were mailed to several locations via the US Postal Service resulting in twenty-two confirmed or suspected cases of anthrax infection. Because the possibility of a terrorist attack using bioweapons is especially difficult to predict, detect, or prevent in a conventional way, it is crucial to find a solution to nullify a microbial attack.
Currently, there is a lack of efficiency with the conventional method and further developments are necessary to achieve higher biocidal efficiency. Moreover, because of the widespread use of antibiotics and the emergence of more resistant and virulent strains of microorganisms, and furthermore bacterial spores have no metabolism and can withstand a wide range of environmental assaults including heat and UV, there is an immediate need to develop alternative sterilization technologies such as photoelectrochemical sterilization using highly efficient photocatalysts.
Wide band-gap semiconductors can act as sensitizers for light-induced redox processes due to their electronic structure, which is characterized at room temperate by a filled valence band and an empty conduction band. Hydroxyl radicals (OH.) generated by the Titania photocatalyst are very potent oxidants and are nonselective in reactivity.
Titania (TiO2) is currently the photocatalyst of choice for most applications, being the most efficient known photocatalyst. Irradiation of a semiconductor, such as TiO2, with light having an energy equal to or greater than the semiconductor material's band gap energy results in the creation of electrons in the semiconductor's conduction band and holes in its valence band. The injection of these electrons and holes into a fluid region surrounding the semiconductor particles causes electrochemical modification of substances within this region. This technology has been used in photocatalytic processes such as the photo-Kolbe reaction in which acetic acid is decomposed to methane and carbon dioxide and the photosynthesis of amino acids from methane-ammonia-water mixtures.
When irradiated TiO2 particles are in direct contact with or close to microbes, the microbial surface becomes the primary target of the initial oxidative attack. In 1985, Matsunaga and coworkers reported that microbial cells in water could be killed by contact with a TiO2—Pt catalyst upon illumination with near-UV light for 60 to 120 min. Later, the same group of workers constructed a practical photochemical device in which TiO2 particles were immobilized on an acetylcellulose membrane. The loss of membrane structure and membrane functions due to the photochemical oxidation was the root cause of cell death when photocatalytic TiO2 particles are outside the cell. It was observed that the extent of killing depended on the structure of the cell wall and was inversely proportional to the thickness. The findings of Matsunaga et al. redirected the attention for sterilization and resulted in an attempt to use this technology for disinfecting drinking water and removing bioaerosols from indoor air environments.
A variety of devices for air purification using Titania for photocatalytic degradation of organic impurities and microbial contaminates have been disclosed. The primary metal oxide for these devices is TiO2. Typically the challenge was to have the impurity or contaminate in contact with the titania surface for a sufficiently long period of time to effectively remove the desired contaminate and often elaborate systems were designed to increase the effective contact time. In all of these cases, an improvement in the photocatalyst efficiency by increasing the efficiency of the TiO2 would greatly enhance the effectiveness of these devices. Moreover, the ability to use photocatalysts for air purification using visible light or sunlight, as opposed to conventionally used UV light, is highly desirable.